[meteorite-list] slices and oxidation
From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:58 2004 Message-ID: <25.42c614b6.2d2af6e2_at_aol.com> --part1_25.42c614b6.2d2af6e2_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Hello list, I have followed the thread regarding the efforts some of the dedicated consciencious members put into slicing. I have a few questions, which I will list as simple statements I think sensible, with the hope that anyone would please comment on them as they apply to preparing slices. I have never prepared a single slice so ask for your guidance. Theoretically, 1. Oxidation is proportional to concentration of reactants (pressure for gas), resident time, temperature. 2. For a solid concentration can be considered exposed surface area. 3. Polishing to mirror finishes, i.e. using the finest grit possible in experienced hands, minimizes the exposed surface area. 4. Evaporation of water depends on surface area of water exposed. 5. Mirror finished maximize the surface area of water by forcing "beads" down to the samllest sizes to be exposed to evaporative processes, and not hidden between micro pits in the slice surface. Also the minimize the fissure areas which by capillary action draw and retain water and other contaminants in the slice 6. Micropits will always be present to any grit size, although they can effectively be minimized by good polishing beyond some threshold which seems to be #1200 from comments. 7. A fresh fusion crust also has excellent anticorrosion properties as it had essentially no micropits or fissures. It is probably much better due to the plugging effect of "imperfections" during formation in the liquid stone phase. 8. Saw marked, broken samples, etc, tend to increase surface area and evaporation times, and there is probably a simple rule of thumb to say, "if the slice is not polished to #X fine, it is basically a broken off specimen surface. 9. Oxidation on even a mirror finish causes pitting since it is not a uniform process due to the composition of the slice and the distribution of water. 10. Using water during polishing will promote oxidation, during the brief time it is present, as this has nothing to do with the person polishing, it is a simple chemical reality, but, 11. On the other hand, the amount in #10 of oxidation for short exposure times followed by "baking out" the water will temporarily arrest the corrosion processes immediately... 12. Thus, a well dried sample polished in water very briefly will suffer oxidation at the molecular level, but so will all slices cut otherwise eventually due to exposure to humidity...even in a desert, unless they are contained perpetually in a non-reactive atmosphere or completely covered with a non-reactive coating. 13. The trick in #12 for polishing in water is to be sure that the oxidation occuring in the short time does not cause pitting greater the pitting present in tolerance of the newly polished surface. Also, to be sure of this, a superfine dry grit can be used to finish pieces after oven drying to get to the same effect as total dry preparation. 14. If a slice begins to show signs of oxidation, the best thing to do is immediately dry buff it with the finest paper to remove the micropitting, and not to allow it to continue. Rusting "accelerates" not because rust attracts more rust magically. It does so simply because it causes pitting and microcavities which retain water. 15. Anything which retains water increases resident time. 16. Initial drying in an oven is not the best route, as temperature increases reaction rates. Pulling vacuum over it is the way to go if you have the equipment. 17. For most samples, an interesting idea is to polish and then immediately develop a thin stable oxidized layer from the native material over the surface which effectively blocks further oxidation. "fine patina". Please comment ! Gracias Doug Dawn Mexico --part1_25.42c614b6.2d2af6e2_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable <HTML><FONT FACE=3Darial,helvetica><HTML><FONT SIZE=3D2 PTSIZE=3D10 FAMILY= =3D"SANSSERIF" FACE=3D"Arial" LANG=3D"0">Hello list, I have followed the thr= ead regarding the efforts some of the dedicated consciencious members put in= to slicing. I have a few questions, which I will list as simple statem= ents I think sensible, with the hope that anyone would please comment on the= m as they apply to preparing slices. I have never prepared a single sl= ice so ask for your guidance.<BR> <BR> Theoretically,<BR> <BR> 1. Oxidation is proportional to concentration of reactants (pressure f= or gas), resident time, temperature.<BR> 2. For a solid concentration can be considered exposed surface area.<B= R> 3. Polishing to mirror finishes, i.e. using the finest grit possible i= n experienced hands, minimizes the exposed surface area.<BR> 4. Evaporation of water depends on surface area of water exposed.<BR> 5. Mirror finished maximize the surface area of water by forcing "bead= s" down to the samllest sizes to be exposed to evaporative processes, and no= t hidden between micro pits in the slice surface. Also the minimize th= e fissure areas which by capillary action draw and retain water and other co= ntaminants in the slice<BR> 6. Micropits will always be present to any grit size, although they ca= n effectively be minimized by good polishing beyond some threshold which see= ms to be #1200 from comments.<BR> 7. A fresh fusion crust also has excellent anticorrosion properties as it ha= d essentially no micropits or fissures. It is probably much better due= to the plugging effect of "imperfections" during formation in the liquid st= one phase.<BR> 8. Saw marked, broken samples, etc, tend to increase surface area and=20= evaporation times, and there is probably a simple rule of thumb to say, "if=20= the slice is not polished to #X fine, it is basically a broken off specimen=20= surface.<BR> 9. Oxidation on even a mirror finish causes pitting since it is not a=20= uniform process due to the composition of the slice and the distribution of=20= water.<BR> 10. Using water during polishing will promote oxidation, during the brief ti= me it is present, as this has nothing to do with the person polishing, it is= a simple chemical reality, but,<BR> 11. On the other hand, the amount in #10 of oxidation for short exposu= re times followed by "baking out" the water will temporarily arrest the corr= osion processes immediately...<BR> 12. Thus, a well dried sample polished in water very briefly will suff= er oxidation at the molecular level, but so will all slices cut otherwise ev= entually due to exposure to humidity...even in a desert, unless they are con= tained perpetually in a non-reactive atmosphere or completely covered with a= non-reactive coating.<BR> 13. The trick in #12 for polishing in water is to be sure that the oxidation= occuring in the short time does not cause pitting greater the pitting prese= nt in tolerance of the newly polished surface. Also, to be sure of thi= s, a superfine dry grit can be used to finish pieces after oven drying to ge= t to the same effect as total dry preparation.<BR> 14. If a slice begins to show signs of oxidation, the best thing to do= is immediately dry buff it with the finest paper to remove the micropitting= , and not to allow it to continue. Rusting "accelerates" not because r= ust attracts more rust magically. It does so simply because it causes=20= pitting and microcavities which retain water.<BR> 15. Anything which retains water increases resident time.<BR> 16. Initial drying in an oven is not the best route, as temperature in= creases reaction rates. Pulling vacuum over it is the way to go if you= have the equipment.<BR> 17. For most samples, an interesting idea is to polish and then immedi= ately develop a thin stable oxidized layer from the native material ov= er the surface which effectively blocks further oxidation. "fine patin= a".<BR> <BR> Please comment !<BR> Gracias<BR> Doug Dawn<BR> Mexico</FONT></HTML> --part1_25.42c614b6.2d2af6e2_boundary-- Received on Mon 05 Jan 2004 12:20:34 PM PST |
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